The present invention relates to apparatus and methods for determining battery state of charge (SoC) and state of health (SoH) and, more particularly, apparatus and methods for determining online battery SoC and SoH in a dynamic charge and discharge environment.
The rechargeable battery is a critical element of recently emerging micro-grid, electric vehicle (EV), plug-in hybrid electric vehicle (PHEV) and more electric aircraft (MEA) systems. These platforms require frequent charging and discharging of the batteries. For reliable operation and to preserve battery life, it is mandatory that accurate knowledge of battery state-of-charge (SoC) and prevailing battery state-of-health (SoH) are known. The prevailing battery capacity is the leading indicator of SoH.
Prior art provides several SoC and SoH determination techniques. However, these techniques can only be applied when the battery is in the off-line mode, i.e., in the laboratory environment, or when it is not being used to support the charge and discharge functions within the host environment. Due to the anomalous behavior of the battery under dynamic charge/discharge conditions, which occur in the online mode, these technique are either inapplicable or cannot provide accurate results.
The most basic technique for determining SoC is based upon the battery open circuit or rested voltage (OCV) measurement. The OCV is typically defined to be the battery terminal voltage after it has been rested at no load or charge for a predetermined time, from a minimum of 30 minutes to several hours. In case of many Li-ion and other battery chemistries, the OCV varies with the SoC and consequently cannot be used to compute the SoC. FIG. 1 provides an example of OCV-SoC correlation for the GS Yuasa 28V, 40Ah Li-ion battery at 0, 25 and 45° C. The Figure indicates a near linear relationship between the SoC and OCV in the approximately 10% SoC to 100% SoC range. The OCV vs. SoC plots are commonly available from the battery/cell suppliers, or can be developed through lab tests. Using this approach, algorithms have been developed with better than +/−5% SoC accuracy. However, this approach is unsuitable for online SoC determination in a dynamic charge/discharge environment where due to the polarization phenomenon, rested battery voltage is rarely available. The polarization phenomenon is illustrated in FIGS. 2A and 2B, which shows battery response to a repetitive 10A load pulse. The inset magnified plot of the battery voltage depicts a sudden jump in voltage upon removal of the load followed by much slower voltage recovery resulting in a long wait time for capturing the rested battery voltage. The wait time is usually not practical in the online environment.
The battery capacity fades with use and calendar time. If a battery's capacity has faded by 50%, at 100% SoC it will only have half the energy compared to when it was first fielded. Therefore, the SoC measurement alone is not sufficient to ensure effective operation of the battery within the system. Battery capacity which the main indicator of SoH should also be monitored.
In the prior art, methods which predict the battery capacity exist, but all these methods can only be implemented offline and require the wait time for battery to reach the equilibrium state. As described in U.S. Pat. No. 7,576,545, the full capacity of the battery can be determined through partially charging/discharging. This method starts with a known SoC state and, after a known amount of energy is added or subtracted, the rested open circuit voltage of the battery is measured to compute the new SoC. The full capacity (Cfull) can then be obtained by correlating the charge/discharge energy (ΔE) with the change in SoC (ΔSoC) by equation (1)Cfull*ΔSoC=ΔE  (1)where ΔSoC=|SoCafter−SoCbefore|. But, after partial charging/discharging, rested open circuit voltage needs be obtained through depolarization/predefined rest time, thus interrupting the system operation. This is not practical when the battery is online.
As can be seen, there is a need for an online technique for determining SoC and SoH of a battery.